Quantifying Direct and Indirect Effects of Elevated CO2 on Ecosystem Response Simone Fatichi Institute of Environmental Engineering, ETH Zürich, Switzerland simone.fatichi@ifu.baug.ethz.ch 26 September, 2017 Garmisch-Partenkirchen, Germany

MOTIVATION CO2 INCREASE IS EXPECTED TO HAVE IMPORTANT CONSEQUENCES! HYDROLOGY ENERGY FLUXES CARBON UPTAKE and STORAGE NUTRIENT DYNAMICS Betts et al. 2007 Nature Schimel et al. 2015 PNAS Introduction Methods Results Conclusions

Leaf Level Physiological Response MOTIVATION Water Use Efficiency Photosynthesis Transpiration CO2 H2O Leaf Level Physiological Response C3 plants Stomatal conductance Net Assimilation CO2 [ppm] CO2 [ppm] Introduction Methods Results Conclusions

LITERATURE CONTEXT (Indirect effects) Hovenden et al. 2014 Nature Morgan et al. 2011 Nature Holtum and Winter 2010 Funct. Plant Biol.

Where should we expect the strongest response? RESEARCH QUESTIONS Which is the proportion of direct and indirect effects of elevated CO2? Where should we expect the strongest response?

PARTITIONING EFFECTS OF CO2 FERTILIZATION E1: DIRECT PHYSIOLOGICAL EFFECT ON PHOTOSYNTHESIS E2: INDIRECT EFFECT THROUGH SOIL MOISTURE SAVINGS E3: INDIRECT EFFECT THROUGH INCREASED LAI E4: INDIRECT EFFECT THROUGH INCREASED LAI AFFECTING SOIL MOISTURE TOTAL CO2 EFFECT = E1 + E2 + E3 + E4 E1 E2 E3 E4 Introduction Methods Results Conclusions

PARTITIONING EFFECTS OF CO2 FERTILIZATION Numerical Simulations 0. CO2 ambient, SM ambient and LAI ambient CO2 elevated, SM elevated and LAI elevated = E1+E2+E3+E4 CO2 elevated, SM ambient and LAI ambient = E1 CO2 elevated and LAI ambient = E1+E2 CO2 elevated, SM ambient and LAI elevated = E1+E3 CO2 elevated, SM elevated and LAI ambient = E1+E2+E4 Ambient: 375 ppm Elevated : 550 ppm Simulations (2 to 5) carried out imposing a given LAI or soil moisture (SM) Introduction Methods Results Conclusions

Tethys-Chloris (T&C) METHOD: MECHANISTIC ECOHYDROLOGICAL MODEL Hydrological Part Tethys-Chloris (T&C) Vegetation Part Fatichi et al., 2012a,b, J. Advances in Modeling Earth Systems Fatichi and Leuzinger 2013, Agr. For. Met. Fatichi et al., 2014, WRR; Fatichi and Ivanov 2014, WRR; Fatichi et al 2016, PNAS Pappas et al. 2016 NP; Paschalis et al. 2015 JGR Fatichi and Pappas, 2017, GRL NO NUTRIENT LIMITATIONS! Introduction Methods Results Conclusions

44 CASE STUDIES COVERING DIFFERENT CLIMATES (Flux towers and manipulation experiments) 3 to 32 years of hourly meteorological forcing Mean Annual Precipitation [mm] Mean Annual Temperature [°C] Introduction Methods Results Conclusions

FACE CASE STUDIES SERC (USA) Florida Scrub Oak ORNL FACE (USA) SWISS CANOPY CRANE (CH) Mixed Deciduous Forest DUKE FACE (USA) Loblolly Pine and Mixed Deciduous TasFACE (AUS) C3/C4 Grassland SERC (USA) Florida Scrub Oak ORNL FACE (USA) Sweetgum plantation Introduction Methods Results Conclusions

DUKE FACE NPP NPP Effect Size [%] [gC m-2 yr-1] Data from: De Kauwe et al., 2013 GCB; Zaehle et al 2014 NP YEAR YEAR Effect Size [%] NPP ET WUE LAI SLA Very consistent predictions across a range of metrics

TasFACE Total CO2 Effect Average across years SIM. 23% OBS. 18% Ambient CO2 Ambient CO2 R=0.82 Biomass [gDM m-2] SIM. Peak Biomass [gDM / m2] YEAR Elevated CO2 OBS. Peak Biomass [gDM / m2] Total CO2 Effect Average across years SIM. 23% OBS. 18% Biomass [gDM m-2] Data from: Hovenden et al., 2014 Nature YEAR Introduction Methods Results Conclusions

WETNESS INDEX = Pr/EPOT NPP – Total Effect Ambient: 375 ppm Elevated : 550 ppm C3 C4/C3 Total Effect on NPP [-] WETNESS INDEX = Pr/EPOT Introduction Methods Results Conclusions

NPP: PARTITIONING EFFECTS E2: indirect soil moisture savings E1: direct E3: indirect increased LAI Effect [-] Effect [-] E4: indirect LAI affecting soil moisture WETNESS INDEX WETNESS INDEX Introduction Methods Results Conclusions

NPP: PARTITIONING EFFECTS Removing the changes in LAI has contrasting effects NPP Effect [-] WETNESS INDEX Total vs. Direct + SM Indirect Introduction Methods Results Conclusions

NPP – Direct vs Indirect NPP Indirect/Direct Effect C4 predominant C3 C4/C3 1.0 NPP Indirect/Direct Effect 0.25 GW fed Trop. Forest Logarithmic Scale WETNESS INDEX Introduction Methods Results Conclusions

EVAPOTRANSPIRATION: PARTITIONING EFFECTS ET Total Effect [-] Effect [-] Effect [-] WETNESS INDEX WETNESS INDEX WETNESS INDEX Introduction Methods Results Conclusions

Inherent Water Use Efficiency (Ecosystem scale) IWUE = GPP*VPD/ET 1.3-1.8 % ppm-1 Keenan et al. 2013 Nature 0.35±0.1 % ppm-1 Cheng et al. 2017, Nat. Comm. IWUE change [%/ppm] WETNESS INDEX Introduction Methods Results Conclusions

Can plant-trait plasticity explain the “unexpected” increase in WUE? RESEARCH QUESTIONS Can plant-trait plasticity explain the “unexpected” increase in WUE?

INHERENT WATER USE EFFICIENCY if is roughly constant Flux-tower observations from 21 forested ecosystem in northern hemisphere (≈1995-2010) Keenan et al. 2013 Nature IWUE [gC/kgH20 hPa] YEAR Expectation of 0.5%/yr increase in IWUE Increase in IWUE of 2.3%/yr vs. Model ensemble simulates 0.04 %/yr Introduction Methods Results Conclusions

DESIGN OF THE EXPERIMENT Simulations with “static” parameters. Re-computing IWUE with longer time series and a proper estimate of uncertainty. Simulations with “static” parameters. Simulations with trends [1% yr-1] in a number of selected plant traits (model parameters). Observations Base Simulation Change in Plant traits IWUE [gC/kgH20 hPa] YEAR Introduction Methods Results Conclusions

Median IWUE change (1.3%/yr). It is 2.6 times above expectations CHANGES IN IWUE IWUE Expectation Frequency Slope [%/yr] SIM with no T and VPD trend SIM Base SIM with plastic traits OBS Kee. 2013 SIM SIM with plasticity Slope [%/yr] Median IWUE change (1.3%/yr). It is 2.6 times above expectations Introduction Methods Results Conclusions

CONCLUSIONS (1) Mechanistic models do provide insights on ecosystem response to elevated CO2. Differences between CO2 scenarios (375 vs 550 ppm) in terms of water fluxes are typically < 8%, while they are in the order of 20-40% for NPP. Indirect effects can be comparable or larger than direct effects. Additional analyses are required to better understand the effect of sink limitations (nutrients, turgor, temperature) on plant growth. Regardless, ecosystems experiencing water stress are expected to be the most responsive in terms of carbon fluxes. Ahlström et al. 2015 Science e.g., Donohue et al. 2013 GRL Introduction Methods Results Conclusions

CONCLUSIONS (2) We computed a median increase in IWUE of 1.3%/yr across 20 north hemisphere forests in the last two decades. T&C predicts an increase of 0.9%/yr in IWUE larger than expected but less than observed. Plasticity in ecosystem traits can potentially explain the “unexpectedly” large IWUE increase. A new challenge for the parametrization of ecosystem models, where key physiological parameters may not be necessary temporally constant, is suggested. Introduction Methods Results Conclusions

Thanks for your attention ! Mastrotheodoros et al. 2017 JGR Fatichi et al. 2016 PNAS Mastrotheodoros et al. 2017 JGR

MOTIVATION PROJECTIONS OF FUTURE TERRESTRIAL CARBON AND WATER CYCLE RELY ON NUMERICAL TOOLS DGVMs Terrestrial Biosphere Models Terrestrial Ecosystem Models Ecohydrological Models RESULTS FROM MANY MANIPULATION EXPERIMENTS ARE AVAILABLE Warming Rainfall Exclusion Girdling Introduction Methods Results Conclusions

PARTITIONING EFFECTS OF CO2 FERTILIZATION NPP [gC / m2 day] DUKE FACE (USA) Drought DAYS E1: direct E2: indirect soil moisture savings dNPP [gC / m2 day] E4: indirect LAI affecting soil moisture DAYS Introduction Methods Results Conclusions

Data from: De Kauwe et al., 2013 GCB; Zaehle et al 2014 NP ORNL FACE NPP NPP [gC m-2 yr-1] Effect Size [%] Data from: De Kauwe et al., 2013 GCB; Zaehle et al 2014 NP YEAR YEAR Nutrient Limitations Effect Size [%] NPP ET WUE LAI SLA Introduction Methods Results Conclusions

EVAPOTRANSPIRATION: PARTITIONING EFFECTS ET Total Effect [-] Effect [-] WETNESS INDEX WETNESS INDEX Introduction Methods Results Conclusions

LITERATURE CONTEXT Novak et al. 2004 New Phyt. Precipitation  ANPP- Ele/Amb [-] Novak et al. 2004 New Phyt. Precipitation  Introduction Methods Results Conclusions

NPP – Direct effect only TOTAL EFFECT on NPP C4 Sites Air temperature [°C] VPD [Pa] Introduction Methods Results Conclusions